Improved compositions comprising a cross-linkable protein or polypeptide, and a non-toxic material which induces cross-linking of the cross-linkable protein. The compositions are optionally and preferably prepared in a non-phosphate buffer solvent. Optionally and preferably, the cross-linkable protein includes gelatin and any gelatin variant or variant protein as described herein. Optionally and preferably, the non-toxic material comprises transglutaminase (TG), which may optionally comprise any type of calcium dependent or independent transglutaminase, which may for example optionally be a microbial transglutaminase (mTG).